[1]up travel is another way to say droop. Its the travel "up" when lifting the suspension before the wheels leave the ground. so 2mm of up travel in the rear means that if your ride height is set at 5mm then when lifting the rear dead center(usually with an exacto knife) both wheels should leave the ground at 7mm. 7mm-5mm = 2mm of up travel. This method is also used as a quick droop adjustment method. If one tire lifts up before the other, most likely your droop is off, or you are tweaked or possibly shock lengths are different. More than likely though your droop is off.
[1]
http://www.rctech.net/forum/electric...up-travel.html
This should help explain... I hope